Ancillary equipment for membrane bioreactors

Two key pieces of additional
equipment required for an MBR which are not required in classical activated
sludge treatment comprise:

fine screening, and

membrane process operation control.

While a membrane bioreactor can
effectively displace primary sedimentation, biotreatment and secondary
solid–liquid separation, as well as tertiary effluent polishing, classical bar
screens of around 6 mm rating are insufficiently selective for an MBR. Such
relatively coarse screens increase the risk of clogging of both membrane
channels and, for immersed systems, the coarse bubble aerators.

Screen ratings for MBRs range from 3
mm for a flat sheet membrane down to below 1 mm for a hollow fibre.

Process control is fundamental to
all wastewater treatment processes. The design approach for MBR control systems
is guided by the application/duty, and in particular the scale, and the
membrane configuration. Large-scale commercial systems require stability of
operation, optimum use of energy and chemicals, and robust control when
challenged with changes in hydraulic and/or organic loadings.

Perhaps most critically, operation
outside of the envelope (specifically permeability and cleaning) stipulated by
the membrane technology supplier may invalidate the warranty.

Equipment terms

Definitions of some common ancillary
equipment are given below.

Degritters

Equipment placed downstream of the
coarse screen at the inlet works to remove coarse, dense particles (primarily
grit) by settlement. They can either be configured as channels or cyclones. At
large plants the grit is normally washed and reused as a building material,
usually for roads.

Screens

Equipment placed at the inlet works
(coarse screens) and downstream of the degritter and/or upstream of the
biological and membrane tanks (fine screen) for removing extraneous rag-like
material by sieving. Coarse screens at the inlet works are normally rated as 6
mm, and the apertures are normally slits for this duty. Fine screens for
protecting the membrane can be rated between 0.8 and 3mm, depending on the
membrane type, and can have slit, square or circular apertures.

Buffer tank

A tank placed upstream of the
biological and membrane tanks to provide buffering, or equalisation, of the
flow and thus reduce hydraulic surges.

Macerators

Equipment used for chopping and grinding bulky and fibrous material in a wide variety of applications in wastewater treatment systems, as well as for mixing and grinding various liquid/solid deposits. They are sometimes used for treating screens to reduce the coarseness of the solids and allow them to be passed through the screen for treatment along with the main flow.

Blowers/compressors

Equipment used for providing air to
the aerators. Wastewater treatment normally employs blowers, which operate at
lower pressure and have a lower energy demand than compressors.

Chemical dosing

Chemical dosing is the application
of chemicals for such applications as pH correction, cleaning (of the membranes
and aerators, in the case of an MBR), and disinfection. A complete chemical
dosing system will normally comprise chemicals storage tanks, dosing pumps and
a system for measuring, monitoring and controlling the dose rate.

Pumps

An apparatus or device for forcing the movement of liquid through equipment.

Sanitisation

Sanitisation applies to any process
which makes the water and/or related equipment sanitary, as by cleaning or
disinfecting to substantially remove pathogenic or problem micro-organisms in
treated water. This is often taken as including biofilms formed on the inner
surfaces of pumps, pipework and fittings, as well as in membrane permeate
channels.

UV irradiation

A process, as applied in water and
wastewater treatment, which provides disinfection through transmitting
ultraviolet irradiation through the water and inactivating micro-organisms. It
is sometimes used downstream of an MBR or membrane process to provide
additional protection against pathogenic bacteria.

Sludge/residuals management

Residuals are those materials
forming the waste products from water and wastewater treatment, which for MBRs
primarily comprises sludge discharged from the membrane and/or biotank.